Fluid capacity of the eye: <= 30 microliters
The optimal volume of an eye drop for drug delivery: 5-10 microliters (Akers, 1983)
Common volumes for 1 eye drop of ophthalmologic medication: 35-75 microliters
Factors affecting the volume of an eye drop:
(1) viscosity of the solution
(2) dropper design (reliable delivery of very small volumes can make the dropper expensive)
(3) patient technique
Absorption of drug in an eye drop depends on:
(1) concentration of the drug in solution
(2) volume of drug that remains in contact with the eye
(3) duration of contact
(4) dilution or displacement of drug by tearing or another eye drop
(5) frequency of administration
The usual period of time that the drug is in contact with the ocular surface is 1-2 minutes.
To increase the duration of drug absorption solution may be formulated to increase the duration of corneal contact (gel, liposomes, etc.).
Patient technique is very important. Patient factors affecting drop delivery:
(1) head and/or hand tremors
(2) noncompliance
(3) inability to coordinate hand and eye activities
(4) blinking
(5) excessive tearing
The solution should be nonirritating. Excessive burning may result in noncompliance, excessive tearing or improper administration.
If 2 different eye drops are used then they should be given at different times to prevent displacement and/or interactions.
(1) The time interval may be specified by manufacturer or physician to reduce interactions.
(2) If not specified then different drops should be instilled at least 5 minutes apart.
(3) Alternatively a combination product can be used if there are no drug interactions.
